Electroless chromium plating process and composition

ABSTRACT

Articles being electroplated with zinc are rapidly coated with a resistant and well adhering electroless chromium plating by immersing the articles in an aqueous bath containing A. 8 TO 15 LITERS OF A SOLUTION OF 250 TO 350 G./L. OF SODIUM DICHROMATE AND 200 TO 300 G./L. OF SODIUM NITRATE, SAID SOLUTION BEING ADJUSTED TO PH .3 with nitric acid, B. 3 TO 8 LITERS OF A SOLUTION OF 500 TO 700 G./L. OF 85 PERCENT FORMIC ACID AND C. .5 TO 3 LITERS OF GLACIAL ACETIC ACID AND CONCENTRATED ACETIC ACID RESPECTIVELY EACH PER 100 LITERS AND MAINTAINING THE PH value of the solution at a pH of 2.1 to 2.7.

United States Patent [191 Lindemann 1 1 ELECTROLESS CHROMIUM PLATING PROCESS AND COMPOSITION [76] Inventor: Karl I-I. Lindemann, Birkenstrasse 13, 6081 Geinsheim, Germany [22] Filed: May 8, 1972 211 Appl. No; 251,409

[52] U.S. CI. 106/1, 117/130 E, 148/62 [51] Int. Cl. C23C 3/00 [58] Field of Search 106/1; 117/130 E; 148/62; 204/51 [56] References Cited I UNITED STATES PATENTS 2,276,353 3/1942 Thompson 148/62 2,393,663 1/1946 Thomas et a1.. 148/62 2,841,540 7/1958 Smith 204/51 3,021,267 2/1962 Berzins 204/51 3,511,759 Nelson 204/51 June 11, 1974 Primary Examiner-Lorenzo B. Hayes Assistant Examiner-Linda Digy Attorney, Agent, or Firm-Erich M. H. Radde [5 7] ABSTRACT Articles being electroplated with zinc are rapidly coated with a resistant and well adhering electroless chromium plating by immersing the articles in an aqueous bath containing a. 8 to 15 liters of a solution of 250 to 350 g./l. of sodium dichromate and 200 to 300 g./1. of sodium nitrate, said solution being adjusted to pH .3 with nitric acid, b. 3 to 8 liters of a solution of 500 to 700 g./1. of 85 percent formic acid and c. .5 to 3 liters of glacial acetic acid and concentrated acetic acid respectively each per 100 liters and maintaining the pH value of the solution at a pH of 2.1 to 2.7.

10 Claims, No Drawings The invention relates to a process for chromatizing articles which have been electroplated with zinc in particular from acid baths, such as sulfuric acid containing plating baths.

It is known to provide zinc orarticles electroplated with zinc with a passivating chromium coating. Thus it is possible to provide zinc and zinc coatings with a black chromium layer (by the so-called black chromium electroplating process) by using electrolytes which contain about 350 g./l. of chromium trioxide Cr 5 g./l. of, barium carbonate and 5 g/l of glacial acetic acid at high current densities of 100 to 200 amps/dm and low temperatures from 12C. to 18C.

The. chromium coatings obtained in this manner con- I tain 20 to 30 percent of chromium oxide but the pro cess is costly and not generally applicable. Further brightening of zinc coatings may be performed by immersing the zinc plated articles in a solution of 100 g./l. of chromic acid, l0 g./l. of sulfuric acid and 5 g./l. of nitric acid. The chromium coatings obtained by this electroless chromium plating process are of a brown color. Frequently the unterlying zinc coating is attacked by this process and it is not possible to obtain chromium coatings which are resistant to wiping and scratching and which are sufficiently adhering in particular on articles being electroplated with zinc from acid baths.

SUMMARY OF THE lNVENTlON It has been found that electroless plated passivating chromium coatings of olive to brown color which are resistant and well adhering may be provided to articles which had been electroplated with zinc by applying an aqueous plating bath containing particular ingredients in a particular concentration under particular condi-- tions such as a specified pH range within very short immersing periods.

It isanobjectof the invention to provide a chromatizing process for articles electroplated with zinc comprising immersing said articles in-an acid, aqueous bath said bath containing per 100 liters a. 8 to liters ofa solution of 250 to 350 g./lv of sodium dichromate and 200 to 300 g./l. of sodium nitrate, said solution being adjusted to a pH of 0.3 with nitric acid,

b. 3 to 8 liters ofa solution of 500 to 700 g./l. of 85 percent formic acid or an equivalent thereof and c. 0.5 to 3 liters of concentrated and or, respectively, glacial acetic acid and maintaining said bath at a pH of from 2.1 to 2.7.

It is a further object of the invention to provide a chromatizing bath for electroless plating of electroplated zinc articles and coatings, said bath containing the ingredients mentioned under a) to c) above and having a pH of from 2.1 to 2.7.

A further object of the invention is to provide a chromatizing process and an electroless plating bath for zinc articles or articles electroplated with zinc said bath containing per 100 liters 4 i a. l l to 1 L5 liters of a solution of about 300 g./l. of

sodium dichromate and 250 g./l. of sodium nitrate, said solution being adjusted to a pH of 0.3 with nitric acid,

b. 5 liters of a solution of formic acid having a density of 1.133 at 20C. and c. 6 to 12 g./l. of glacial acetic acid, said bath having a pH from 2.1 to 2.7.

Further objects of the invention may be taken from the following description.

DESCRIPTION OF THE INVENTION It has been found that articles of zinc and articles electroplated with zinc, in particular from sulfuric acid containing electroplating baths may be chromatized by applying to said article such an electroless plating bath, thereby carefully adjusting the pH range as mentioned above. Such adjustment may be effected by continuous pH measurement and, if desired, by the addition of fresh solutions to the exhausted bath. The process yields passivating chromium coatings of olive to brown color, having excellent resistance to cracking and wiping and adhering properties within a very short residence time in the bath. The color is mainly governed by the amount of acetic acid in the electroless chromium plating bath and is reproducible. The residence time in the bath amounts to less than 60 seconds such as- 10 seconds or less. The resulting passivating chromium layer is completely resistant against wiping and scratching in the wet state and is resistant to cracks re sulting from bendingthe article. It is well adhering in the drystate in particular at upset zones of zinc plated articles which have been provided with a chromium layer. This is of'particular importance with articles electroplated with zinc and provided with an electroless chromiurn layer which articles are bent sub-- EXAMPLE A solution A consisting of 30 kg. of sodium dichromate and 25 kg. of sodium nitrate, filled up to 100 liters with deionized water and adjusted to a pH of 0.3 with nitric acid was prepared. The density of the solution amounted to l.326 at 20C.

Further, a solution B consisting of 65.8 kg. (53.8 liters) of percent formic acid, filled up to liters with deionized water was prepared. The density of the solution amounted to 1.133 at 20C.

1 1.2 liters of solution A, 5 liters of solution B and 690 cm of 98 percent acetic acid were filled up to 100 liters using deionized water.

In the electroless chromium plating bath thus obtained there were immersed iron articles provided with an electroplated zinc coating obtained from a sulfuric acid containing electrolyte. After 13 seconds of residence time the articles were taken from the bath. There was obtained a finely crystalline chromium coating of olive color which was resistant against wiping and scratching in the wet state and which adhered very What is claimed is:

1. In a process of chromatizing zinc articles and zinc electroplated articles, the step which comprises immersing said articles in an acid aqueous chromatizing bath for a period of time less than 60 seconds, said bath containing per 100 liters a. 8 liters to 15 liters of an aqueous solution of 250 g./l. to 350 g./l. of sodium dichromate and 200 g./l. to 300 g./l. of sodium nitrate, said solution being adjusted to a pH of 0.3 by the addition of nitric acid; b. 3 liters to 8 liters of an aqueous solutionof 500 g./l. to 700 g./l. of 85 percent formic acid or equivalent amounts thereof, and v c. 0.5 liters to 3 liters of glacial acetic acid or concentrated acetic acid, the pH-value of said bath being maintained at a pH between 2.1 and 2.7.

2. The chromatizing process of claim 1, in which the zinc electroplated articles to be chromatized are articles that have been electroplated from a bath comprising sulfuric acid and zinc.

3. The chromatizing process of claim 1, in which the chromatizing bath is a bath containing per 100 liters a. l 1.0 liters to 11.5 liters of an aqueous solution of about 300 g./l. of sodium dichromate and 250 g./l. of sodium nitrate, said solution being adjusted to a pH of 0.3 by the addition of nitric acid,

b. 5 liters of an aqueous formic acid solution of a density of 1.133 at 20 C., and

c. 6 g./l. to 12 g./l. of glacial acetic acid, the pH-value of said bath being maintained at a pH between 2.1 and 2.7. i

4. An electroless chromium plating bath comprising an aqueous solutioncontaining per 100 liters a. 8 liters to 15 liters of an aqueous solution of 250 g./l. to 350 g./l. of sodium dichromate and 200 g./l. to 300 g./l. of sodium nitrate, and nitric acid in an v amount sufficient to adjust the pH of the solution b. 3 liters to 8 liters of an aqueous solution of 500 g./l. to 700 g./l. of percent formic acid or equivalent amounts thereof, and

c. 0.5 liters .to 3 liters of glacial acetic acid or concentrated acetic acid,

said bath having a pH between 2.1 and 2.7.

5. The electroless chromium plating ,bath of claim 4, said bath comprising an aqueous solution containing per liters a. 11.0 liters to 1 1.5 liters of an aqueous solution of about 300 g./l. of sodium dichromate and 250 g./l. of sodium nitrate, and in an amount sufficient to adjust the pH of the solution to 0.3,

b. 5 liters of an aqueous formic acid solution of a density of 1.133 at 20 c., and

. c. 6 g./l. to 12 g./1. of glacial acetic acid,

said bath having a pH between 2.1 and 2.7.

6..An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim 1.

7. An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim 2.

8. An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim 3.

9. A chromatized article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by immersing the article in the electroless chromatizing bath of claim 10. A chromatized article with a zinc surface, said article beingprovided with a chromium layer deposited on said zinc surface in an electroless manner by immersing the article in the electroless chromatizing bath of claim 5.

22 3 UN1TED sm'r -s PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3 -2 Dated I June 11, 1974 Inventofls) KARL H. LINDEMANN It is certified that error appears in the above-identified pateht and that said Letters Patent are hereby corrected as shown below:

I After [21] Appl. No. 251,409" there should be inserted:

- Claims priority, application Germany, January 19, 1972, P 22 02 457.3

Abstract of the Disclosure, line 7: 3" should read Abstract of the Disclosure, line 11: ".5" should read Column 4, line 14: After "and" there should be inserted nitric acid Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON J'R. c. MARSHALL DANN Arresting Officer Commissioner of Patents 

2. The chromatizing process of claim 1, in which the zinc electroplated articles to be chromatized are articles that have been electroplated from a bath comprising sulfuric acid and zinc.
 3. The chromatizing process of claim 1, in which the chromatizing bath is a bath containing per 100 liters a. 11.0 liters to 11.5 liters of an aqueous solution of about 300 g./l. of sodium dichromate and 250 g./l. of sodium nitrate, said solution being adjusted to a pH of 0.3 by the addition of nitric acid, b. 5 liters of an aqueous formic acid solution of a density of 1.133 at 20* C., and c. 6 g./l. to 12 g./l. of glacial acetic acid, the pH-value of said bath being maintained at a pH between 2.1 and 2.7.
 4. An electroless chromium plating bath comprising an aqueous solution containing per 100 liters a. 8 liters to 15 liters of an aqueous solution of 250 g./l. to 350 g./l. of sodium dichromate and 200 g./l. to 300 g./l. of sodium nitrate, and nitric acid in an amount sufficient to adjust the pH of the solution to 0.3; b. 3 liters to 8 liters of an aqueous solution of 500 g./l. to 700 g./l. of 85 percent formic acid or equivalent amounts thereof, and c. 0.5 liters to 3 liters of glacial acetic acid or concentrated acetic acid, said bath having a pH between 2.1 and 2.7.
 5. The electroless chromium plating bath of claim 4, said bath comprising an aqueous solution containing per 100 liters a. 11.0 liters to 11.5 liters of an aqueous solution of about 300 g./l. of sodium dichromate and 250 g./l. of sodium nitrate, and in an amount sufficient to adjust the pH of the solution to 0.3, b. 5 liters of an aqueous formic acid solution of a density of 1.133 at 20* c., and c. 6 g./l. to 12 g./l. of glacial acetic acid, said bath having a pH between 2.1 and 2.7.
 6. An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim
 1. 7. An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim
 2. 8. An article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by a process as claimed in claim
 3. 9. A chromatized article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by immersing the article in the electroless chromatizing bath of claim
 4. 10. A chromatized article with a zinc surface, said article being provided with a chromium layer deposited on said zinc surface in an electroless manner by immersing the article in the electroless chromatizing batH of claim
 5. 